Skin cancer is the most common human cancer. In 1999, it is estimated that there will be 70000 new cases of skin cancer in Canada (Canadian Cancer Statistics: Toronto: National Cancer Institute of Canada, 1999) and more than 1 million new cases in the United States. The clinical diagnosis is often difficult since many benign skin diseases resemble malignancies upon visual examination. As a consequence, histopathological analysis of skin biopsies remains the standard for confirmation of a diagnosis. However, the decision must be made as to which and how many suspicious skin diseases to biopsy.
A rapid, non-invasive technique that could be utilized for characterization of skin diseases prior to biopsy would be useful. Visible/infrared (IR) spectroscopy may be that tool (Jackson et al, 1997, Biophys Chem 68:109-125). The IR spectrum is divided into three regions: near-IR (700-2500 nm), mid-IR (2500-50000 nm) and far-IR (beyond 50000 nm). As light in the far-IR region is completely absorbed by tissues, it is of little use for tissue analysis. Mid-IR light is absorbed by a variety of materials in skin, thus providing an insight into skin biochemistry. We have shown that biopsies from basal cell carcinoma (BCC), squamous cell carcinoma (SCC) and melanocytic tumors have distinct mid-IR signatures when compared to normal skin (McIntosh et al, 1999, J Invest Dermatol 112:951-956; McIntosh et al, 1999, Biospectroscopy 5:265-275; Mansfield et al, 1999; Appl Spectroscopy, 53:1323-1330). However, the diagnostic potential of mid-IR spectroscopy in-vivo is limited, since complete absorption of mid-IR light results with samples greater than 10-15 μm in thickness. In contrast, near-IR light is scattered to a much greater extent than it is absorbed, making tissues relatively transparent to near-IR light, thus allowing the examination of much larger volumes of tissue and the potential for in-vivo studies.
The near-IR region is often sub-divided into the short (680-1100 nm) and long (1100-2500 nm) near-IR wavelengths, based upon the technology required to analyze light in these wavelength regions. At shorter near-IR wavelengths, the heme proteins (oxy- and deoxyhemoglobin and myoglobin) and cytochromes dominate the spectra, and their absorptions are indicative of regional blood flow and oxygen consumption. Long wavelength near-IR absorptions arise from overtones and combination bands of the molecular vibrations of C—H, N—H and O—H groups. The absorption of near-IR light therefore provides information concerning tissue composition (i.e. lipids, proteins) and oxygen delivery and utilization.
Acquisition of visible/near-IR data is straightforward. Visible and near-IR light is brought from a spectrometer to the skin via a fiber optic cable. The light penetrates the skin, and water, hemoglobin species, cytochromes, lipids and proteins absorb this light at specific frequencies. The remaining light is scattered by the skin, with some light being scattered back to the fiber optic probe. The light is collected by the probe and transmitted back to the spectrometer for analysis. A plot of the amount of light absorbed at each wavelength (the spectrum) is computed. Measurements are rapid, non-destructive and non-invasive.